1. Field of the Invention
The present invention relates generally to an X-ray computed tomography (CT) apparatus. More particularly, in a multi-slice X-ray CT apparatus that uses a plurality of rows of detectors for detecting an X-ray image formed by helical scanning of the surroundings of a subject, this invention relates to a technique for achieving data interpolation and image reconstruction based on data obtained from helical scanning of the subject while the patient couch or gantry is tilted. Specifically, the invention relates to the reconstruction of CT scans for a tilted gantry or tilted couch implementing a 3-D Feldkamp cone-beam based reconstruction algorithm.
2. Discussion of the Background
There have previously been proposed X-ray CT devices using a helical scanning system. As illustrated in FIG. 5, the X-ray CT apparatus based on helical scanning collects tomographic image data of a subject to be examined by moving a patient couch in a body axial direction of the subject 12, along the Z axis, in synchronism with a continuous rotation of both an X-ray beam generation source (focus) 13 and a detector 11. Therefore, in the helical scanning system, the X-ray CT apparatus moves the patient couch in a body axial direction of the subject 12 through a center of the rotation of the X-ray focus 13 and the detector 11 while rotating these units. Through this process, the X-ray focus 13 and the detector 11 take a spiral locus around the subject 12 as illustrated in FIG. 4.
There are two types of helical scanning devices: single slice and multi-slice. A single slice CT device has an X-ray beam generation source for irradiating fan shaped X-ray beams (hereafter referred to as fan beams) and a detector having M channels (for example 1,000 channels) arrayed in a fan shape or in a linear shape in one row. This single slice CT device has an X-ray beam generation source and a detector that rotate around the subject. M data is collected in one rotation. Data collection for a single time period is referred to as “one view.”
The second type of helical scanning device, a multi-slice CT scanning device, has an X-ray beam generation source for irradiating conical X-ray beams (hereafter referred to as cone-beams), and a two dimensional detector having detectors arrayed in a body axis (Z axis) direction in a plurality of rows, each detector having an arcuate array of M channel detectors (M channels times N rows). The multi-slice CT device rotates the X-ray focus and the detector around the subject, and collects M×N data in one rotation. Accordingly, as compared with the single-slice CT apparatus, it is possible to scan over a wide range with higher precision and higher speed.
Image reconstruction processing in a helical scanning system includes several steps, as illustrated in FIGS. 6A and 6B. First, as shown in FIG. 6A, projection data collected by the detector at each view of the helical scanning is collected for all angles. The projection data is corrected by taking into consideration the sensitivity of the detector, the X-ray intensity, and various other physical factors. The data after correction is referred to as raw data.
The raw data is then interpolated to generate interpolated data on a desired slice surface. This type of interpolation is known as helical interpolation. The interpolation may be performed using adjacent interpolation, as disclosed for example in Japanese Laid-open Publication Hei 4-224736. Filter interpolation for the addition of weighted multi-point data may also be performed using techniques similar to those described in Japanese Laid-open Publication Hei 9-234195. Other interpolation methods, such as opposite beam interpolation and the like may also be used.
The interpolated data for the respective angles are then subjected to a convolution calculation, as illustrated in FIG. 6B. The convolution calculation uses a filter function selected based on the properties of the X-ray CT device.
The convolved data is then added to all the pixels arrayed along the path of an X-ray beam at the time of data collection. This process is known as backprojection. The backprojection is repeated for all of the convolved data at angles according to the beam shape, thereby leaving only the original signal.
In order to reconstruct the original signal, fan beam reconstruction has often been used, especially in the case of a tilted gantry or tilted patient couch. In order to prevent unnecessary patient exposure to X-rays during CT scans, it is useful to tilt the gantry about the patient or to tilt the couch during scanning. Without compensation for artifacts introduced during the tilting of the gantry or couch, the quality of reconstructed images is poor. In the past, fan beam reconstruction of tilted gantry CT scans has been performed in order to correct for artifacts introduced during the tilt.
However, fan beam reconstruction is an approximation useful for small cone angles associated with multi-slice systems of four or less slices. For larger multi-slice systems, such as those having eight or sixteen slices, the fan beam approximation becomes less and less accurate. As a result, unacceptable artifacts such as loss of resolution and inaccurate structure reconstruction occur.
In previous methods using the ASSR method (described, for example, in M. Kachelriess, T. Fuchs, S. Schaller, and W. A. Kalendar, “Advanced Single-Slice Rebinning for Tilted Spiral Cone-Beam CT,” Med. Phys. 28(6), pp. 1033-1041 (June 2001), the contents of which are herein incorporated by reference), it was not possible to compensate for arbitrary pitch values. Additionally, for incorporating the tilt of the gantry, ASSR requires complete reformulation of the reconstruction problem.